Substituted phenylboronic acids, for example cyanophenylboronic acids, are of considerable-industrial importance as precursors for active compounds, in particular as precursors for correspondingly substituted biphenyl derivatives, which are used as AT(II) antagonists, or as precursors for liquid-crystalline compounds, as liquid crystals or as a constituent of liquid-crystalline mixtures. Phenylboronic acids can be coupled to haloaromatic compounds with transition-metal catalysis to give biphenyl derivatives with the aid of methods described in the literature (N. Miyaura et al., Tetrahedron Lett., 3437 (1979); A. L. Casalnuovo et al., J. Amer. Chem. Soc. 112, 4324 (1990), N. Miyaura et al., Chem. Rev. 95 (1995), 2457-2483).
The conventional synthetic routes for cyanophenylboronic acids, either starting from carboxyphenylboronic acid via the formation of the acid amide with subsequent formation of the cyano compound or starting from the correspondingly substituted bromobenzonitrile by reaction with organolithium compounds, such as butyllithium, followed by reaction with a trialkyl borate, do not achieve the object of an economical synthesis of cyanophenylboronic acids which is simple to carry out industrially, since firstly the synthetic route contains too many steps, and secondly, organolithium compounds are very expensive and hazardous to handle.
The Grignard reaction with chlorobenzaldehyde proceeds in low yields and very slowly, meaning that for industrial purposes, it was hitherto necessary to use expensive bromobenzaldehyde (H. Jendralla et al., Liebigs Ann. 1995, 1253-1257).
WO 98/02 443 uses transition-metal compounds, if necessary in combination with co-catalysts, for activating aromatic chlorine compounds for Grignard reactions, but not for chlorinated aromatic aldehydes or protected derivatives thereof. Rather, it is known that ether and acetal protecting groups considerably reduce the reactivity of the magnesium by forming complexes at the magnesium surface (D. E. Pearson et al., J. Org. Chem., 1959, 24, 504-509).
Owing to the interest in this class of substances, there is a need for an economical synthesis of substituted phenylboronic acids, in particular of cyanophenylboronic acids, which is simple to carry out industrially.

in which
Q1 and Q2 are each OH or together are a divalent radical of the formula (Ib) 
Z is xe2x80x94CHO, D, xe2x80x94CH2Y or X, where D is a protected aldehyde group, Y is hydroxyl or amino, and X is cyano, COOH, COCl, CONH2 or C(OR)3, where R is C1-C5-alkyl or phenyl, and where Z is in the ortho-, meta- or para-position to the boronic acid radical;
R1 to R4, independently of one another, are hydrogen, C1-C12-alkyl, C2-C12-alkenyl, C2-C12-alkynyl, C3-C12-cycloalkyl, (C1-C12-)-alkoxy, O-phenyl, O-benzyl, aryl, heteroaryl, fluorine, N(alkyl)2, N[Si(C1-C4-alkyl)3]2 or CF3, or R1 and R2, and/or R3 and R4, together form a 5- or 6-membered aliphatic or aromatic ring; which comprises
a) reacting a compound of the formula (II) 
xe2x80x83with magnesium in the presence of
i) an anthracene compound and, if desired, a transition-metal halide and, if desired, a magnesium halide; or
ii) a transition-metal halide and, if desired, a magnesium halide, where the anthracene compound is a compound from the group consisting of anthracene, Mg anthracene, substituted anthracene and substituted Mg anthracene,
to give an arylmagnesium chloride of the formula (III) 
b) reacting the compound of the formula (III) with a borate of the formula B(ORxe2x80x2)3, in which Rxe2x80x2 are identical to or different from one another and are straight-chain or branched (C1-C8)-alkyl radicals, phenyl radicals which are unsubstituted or substituted by one or two (C1-C4)-alkyl groups or (C1-C4)-alkoxy groups, in particular straight-chain or branched (C1-C4)-alkyl radicals or unsubstituted phenyl radicals, and hydrolyzing the product to give a compound of the formula (IV) 
xe2x80x83in which
D1 is CHO or D;
Q1 and Q2 are each OH or together are a divalent radical of the formula (IVb) 
c) if desired oxidizing the compound of the formula (IV) or (IVb) in which D1 is CHO to give a compound of the formula (I) in which Z is X, or if desired reducing the compound of the formula (IV) or (IVb) to give a compound of the formula (I) in which Z is CH2Y.
In the above definitions, alkyl is preferably C1-C4-alkyl, aryl is preferably phenyl, alkylaryl is preferably benzyl, and alkoxy is preferably C1-C4-alkoxy.
Preferred radicals R (Z is xe2x80x94C(OR)3) are C1-C4-alkyl, in particular methyl, ethyl or phenyl.
Preferred radicals R1 to R4 are hydrogen, methyl, ethyl, propyl, butyl, methoxy, ethoxy, propoxy, butoxy and fluorine.
The radical D is preferably an acetal of the formula (V) or (VI) 
in which R5 to R8 are identical or different and are hydrogen, C1-C12-alkyl or phenyl, or R6 and R7 together form a 5- or 6-membered aliphatic or aromatic ring; or D is an oxazolidine of the formula (VII) or an oxazoline of the formula (VIII) 
in which R5 to R8 are as defined above, and R9 is C1-C6-alkyl, phenyl or benzyl, unsubstituted or substituted on the aromatic ring.
It was surprising that compounds of the formula (I) can be prepared in good yields by the process according to the invention starting from ortho-, meta- or para-chlorobenzaldehyde,.
Preferred borates B(ORxe2x80x2)3 are trimethyl borate, triethyl borate, tri-n-propyl borate, triisopropyl borate, tri-n-butyl borate and triisobutyl borate.
The group D is, if desired, converted into a compound of the formula (I) in which Z is xe2x80x94CHO by acidic hydrolysis or (in the case of the oxazolines) by reduction followed by acidic hydrolysis. It is also possible to remove the aldehyde protecting group in a one-pot process and, without prior isolation of a compound of the formula (IV) in which D1 is D, to obtain a compound of the formula (IV) in which D1 is xe2x80x94CHO.
It is likewise possible, by reacting compounds of the formula (IV) with alcohols of the formula HO-(C1-C12)-alkyl, HO-(C2-C12)-alkenyl, HO-(C2-C12)-alkynyl, HO-aryl or HO-alkylaryl, to prepare acyclic boronates of the formula (IVa) 
in which Q3 and Q4 are a radical of said alcohols, or, by reaction with the polyhydric alcohols (C3-C12)-cycloalkane-1,2-diol, (C5-C12)-cycloalkene-1,2-diol, (C5-C12)-cycloalkane-1,3-diol, (C5-C12)-cycloalkene-1,3-diol or with the alcohols of the formulae (1) to (6) 
in which R1a to R8a, independently of one another, are hydrogen, C1-C12-alkyl, C1-C12-hydroxyalkyl, C2-C12-alkenyl, C2-C12-alkynyl, C3-C12-cycloalkyl, (C1-C12)-alkoxy, O-phenyl, O-benzyl, aryl, heteroaryl, fluorine, chlorine, NH2, NH(alkyl), N(alkyl)2, N[Si(C1-C4-alkyl)3]2 or CF3, and/or two adjacent radicals R1a to R8a together form a 5- or 6-membered aliphatic or aromatic ring, and in which n is an integer from 2 to 12,
to prepare a cyclic borate of the formula (IVa) in which Q3 and Q4 together are a divalent radical of said polyhydric alcohols.
The compounds of the formula (IVa) can be converted back into compounds of the formula (IV) by acidic hydrolysis.
The compounds of the formula (I) in which Z is CHO, X or xe2x80x94CH2Y can likewise be converted into the compounds of the formula (Ia) by reaction with the above-mentioned alcohols. 
The compounds of the formula (Ia) can be converted back into compounds of the formula (I) by acidic hydrolysis.
The reaction of the compound of the formula (I) or of the formula (IV) with the alcohols on which the radicals Q3 and Q4 are based is advantageously carried out in the presence of an organic solvent which is inert toward the reaction participants, such as tetrahydrofuran, methyl tert-butyl ether, toluene, o-, m- or p-xylene, hexane or heptane, at a temperature of from 20xc2x0 C. to the boiling point of the solvent used. In the case of diols or other polyhydric alcohols, it is also possible to use methanol, ethanol, n- or isopropanol as inert solvent. The diol on which the radicals Q3 and Q4 are based is advantageously employed in an equimolar amount, based on the boronic acid.
Preferred radicals Q3 and Q4 are xe2x80x94O-(C1-C6)alkyl, xe2x80x94O-(C2-C6)alkenyl, xe2x80x94O-(C3-C6)-alkynyl, xe2x80x94O-phenyl, xe2x80x94O-benzyl, or Q3 and Q4, together with the boron atom, form a cyclic boronate with the alcohols ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 2,2-dimethylpropane-1,3-diol, pyrocatechol, pinacol, 2,3-dihydroxynaphthalene, 1,2-dihydroxycyclohexane, 1,3-dihydroxycyclopentane or 1,2-dihydroxycyclooctane.
Particularly preferred radicals Q3 and Q4, together with the boron atom, form a cyclic boronate with the alcohols ethylene glycol, 1,3-propanediol, 2,2-dimethyl-1,3-diol, pinacol and pyrocatechol.
The trimeric compounds of the formula (Ib) or (IVb) can be prepared from the corresponding monomeric compounds of the formula (I) or (IV) respectively, for example by heating at from 40 to 100xc2x0 C., preferably from 50 to 75xc2x0 C.
The process according to the invention is shown in Scheme 1 below: 
The aldehyde group is firstly converted into a magnesium-unreactive form, for example into a cyclic or a cyclic acetal, preferably ethylene glycol acetal, dimethyl or diethyl acetal, an oxazolidine or an oxazoline.
Chlorobenzaldehydes can be reacted with 1,2-diols by conventional methods to give correspondingly substituted 1,3-dioxolanes of the general formula (V) or with trialkyl ortho-esters, such as trimethyl orthoformate, triethyl orthoformate, triisopropyl orthoformate or corresponding orthoacetates, to give acyclic acetals of the general formula (VI). Preference is given here to the reactions with ethylene glycol, pyrocatechol, trimethyl orthoformate, triethyl orthoformate or triisopropyl orthoformate.
Chlorobenzaldehydes can be reacted with 1,2-aminoalcohols which are monosubstituted on the nitrogen to give correspondingly substituted oxazolidines of the general formula (VII) by azeotropic distillation of the water of reaction (T. H. Fife, L. Hagopian, J. Am. Chem. Soc. 1968, 1007-1014). Preferred aminoalcohols are N-methyl-2-aminoethanol, N-ethyl-2-aminoethanol, N-propyl-2-aminoethanol, N-butyl-2-aminoethanol, N-phenyl-2-aminoethanol, N-benzyl-2-aminoethanol, N-methyl-2-aminopropanol, N-ethyl-2-aminopropanol, N-propyl-2-aminopropanol, N-butyl-2-aminopropanol, particularly preferably N-ethyl-2-aminoethanol, N-butyl-2-aminoethanol, N-phenyl-2-aminoethanol, N-benzyl-2-amino-ethanol.
It is furthermore possible to react the correspondingly substituted chlorobenzoyl chloride with 1,2-aminoalcohols by a method described in J. Org. Chem. 1988, 53, 345-352, to give oxazolines. 
Preference is given here to 2-amino-2-methylpropan-1-ol and 2-aminoethanol.
The compound of the formula (II) is converted in accordance with the invention into the Grignard compound of the formula (III) using Mg powder or turnings in the presence of
i) an anthracene compound or ii) an anthracene compound and a transition-metal halide or iii) an anthracene compound and a magnesium halide or iv) an anthracene compound, a transition-metal halide and a magnesium halide or v) a transition-metal halide or vi) a transition-metal halide and a magnesium halide.
The anthracene compounds employed can be unsubstituted anthracene or Mg anthracene, substituted, for example by 1 to 4 (C1-C4)-alkyl groups or phenyl groups, anthracene or Mg anthracene, in particular 9,10-diphenylanthracene or Mg 9,10-diphenylanthracene. The anthracene compounds can be added in amounts of from 0.5 to 100 mol %, preferably from 1 to 10 mol %, based on the haloaromatic compounds, or alternatively formed in situ.
The transition-metal halides are preferably chlorides or bromides, in particular FeCl2, MnCl2, FeBr2 or MnBr2. The transition-metal halides can be added in amounts of from 0.5 to 100 mol %, preferably from 1 to 10 mol %, based on the haloaromatic compounds.
Suitable magnesium halides are MgCl2 and MgBr2. They can be added in amounts of from 0.5 to 100 mol %, preferably from 1 to 10 mol %, based on the haloaromatic compounds.
The Grignard reaction is preferably carried out at the boiling point of the corresponding solvent and under a protective-gas atmosphere. Suitable solvents are usually tetrahydrofuran, diethyl ether, monoglyme and diglyme and a solution of N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine in toluene. It may be advantageous before commencement of the reaction to activate the magnesium by a method described in Y.-H. Lai, Synthesis 585-604 (1981) or to carry out the Grignard reaction in the presence of small amounts, for example from 0.01 to 10 mol %, preferably from 0.1 to 1 mol %, based on the haloaromatic compounds, of a haloalkane, such as, for example, 1,2-dibromoethane, bromoethane or iodomethane.
The compound of the formula (III) is novel and is likewise a subject-matter of the present invention. The compound of the formula (III) can be isolated by removing the solvent by distillation under a protective-gas atmosphere.
In order to obtain phenylboronic acids, the compound of the formula (III) is reacted, preferably without interim isolation, with the borate of the formula B(ORxe2x80x2)3, in particular with B(OCH3)3, B(OEt)3 or B(OiPr)3, and subsequently hydrolyzed under aqueous conditions to give a compound of the formula (IV). The reaction with the borate is advantageously carried out at a temperature of from xe2x88x9280xc2x0 C. to +20xc2x0 C., preferably from xe2x88x9250xc2x0 C. to +10xc2x0 C., in particular from xe2x88x9225xc2x0 C. to 0xc2x0 C.
The borate is advantageously employed in a 1- to 1.5-fold molar amount, based on the Grignard compound.
The compound of the formula (IV) can subsequently be hydrolyzed under acidic conditions, for example using sulfuric acid at pH 0 to 3. If D is an acetal or oxazolidine group, a preferred procedure is, when the addition of the borate is complete, to add the reaction mixture to ice-water and to set the pH of the suspension to from 1 to 2, for example using sulfuric acid, giving the compound of the formula (IV) in which D1 is CHO and Q1 and Q2 are each OH.
Oxazolines, i.e. D is a radical of the formula (VIII), can be converted into the aldehyde by a method described in J. Am. Chem. Soc. 1983, 105, 1586-1590, by alkylation on the nitrogen using alkyl halides, for example methyl iodide, methyl bromide, ethyl iodide or ethyl bromide, or using dialkyl sulfates, for example dimethyl sulfate or diethyl sulfate, hydrogenation using complex metal hydrides, such as LiAIH4, NaBH4 or NaBH3(CN), followed by acidic hydrolysis. 
During the hydrolysis, the aldehyde predicting group is removed and the boronate is converted into the free boronic acid. The formylphenylboronic acid of the formula (I) in which Z is CHO can be isolated from the organic phase of the reaction mixture.
Secondary products can be prepared as shown in Scheme 2 by oxidation or reduction of the aldehyde. In this scheme, Q10 and Q20 are Q1 and Q2, or Q3 and Q4 if the formylphenylboronic acid is esterified, before the oxidation or reduction, by the above-described method using an alcohol on which the radicals Q3 and Q4 are based, in an inert organic solvent. This is particularly advantageous if oxidation is subsequently carried out. 
a) From formylphenylboronic acid and its borates, the corresponding cyanophenylboronic acid of the formula (I) in which Z is CN is obtained by reaction with hydroxylamine or hydroxylammonium salts followed by dehydration of the oxime formed. The dehydration can be carried out by heating in glacial acetic acid or in acetic anhydride (J. Chem. Soc. 1933, IX, 43). By a method proposed in EP-A1-0 790 234, the nitrile function is obtained by reaction of the benzaldehyde derivative with hydroxylamine sulfate in the presence of a tertiary amine base and azeotropic distillation of the water of reaction.
b) The corresponding carboxyphenylboronic acid of the formula (I) in which Z is COOH can be prepared by oxidation of formylphenylboronic acid using barium permanganate or potassium permanganate, for example in accordance with U.S. Pat. No. 5,631,364.
c) The compound of the formula (I) in which Zxe2x95x90CH2OH can be obtained by reduction using Raney nickel/hydrogen or using complex metal hydrides, such as LiAIH4 or NaBH4.
d) The compound of the formula (I) in which Zxe2x95x90COCl can be obtained by a method described in Ginsburg, D., J. Amer. Chem. Soc., 1951, 73, 702-704, by reaction of formylphenylboronic acid with t-BuOCl in carbon tetrachloride.
Starting from cyanophenylboronic acid or borates, further secondary products can be prepared in accordance with Scheme 3.